Vertical sweep synchronizing circuit



June 24, 1952 Filed June 30, 1949 B. M. OLIVER 2,601,415

VERTICAL SWEEP SYNCHRONIZING CIRCUIT 2 SHEETSSHEET l FIG. I 0 '/3 /5sou/v0 LE 4 sou/v0 AME DEFLECWON roxs /4 /6 n. FAME PICTURE 'IWVERTEAAM? '25: PICTURE a osc & 0E7. TUBE d8 L LowPAss 24 STR/PPER I FILTER aimaim 27 FHA! MIXER on AND FULL WAVE RECTIFIER T LOW/1455 RACT4IL ICE 2?25 T08 FILTER I \22 VERI SYNCH PULJ' E CLIFF/N6 LEVEL AFTER CLIFF/N6lNVENTOR By B M OLIVER ATTORNEY June 24, 1952 B. M. OLIVER 2,601,415

VERTICAL SWEEP SYNCHRONIZING CIRCUIT Filed June 30, 1949 2 SHEETSSHEET 2FIG. 2

RECEIVED SYNC- .SIGNAL 1 .mvc. PULSE r0 HOR-SAWTOOTH saw. I 26 lNl/ENTORB M OLIVER ATTORNEV Patented June 24, 1952 VERTICAL SWEEP sYnonnoNrzrNoCIRCUIT Bernard M. Oliver, Morristown, N. 5., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New York-Application June 30, 1949, Serial No. 102,298

1 3' Claims.

This invention relates to television and more specifically to thesynchronization of the horizontal and vertical sweep circuits intelevision receivers.

An object of this invention is to produce more accurate verticalsynchronizing in television receivers.

Another object of this invention is to produce more accurate interlacingof the vertical sweep circuits in television receivers.

Still another object of this invention is to minimize the effect ofnoise, generally present in television receivers, on the verticalsynchronization.

Noise in a received television signal disturbs the receivedsynchronizing pulses and makes it impossible to detect with certaintythe exact position of these pulses. If, for example, the sweep circuitsare arranged to trip when the leading edge of the synchronizing pulsereaches a certain height, then, because the leading edge does not havevand infinite slope, the added noise causes the circuit to trip too earlyor too late in a random dispersion about the correct time. Horizontalsweep circuits, known as AFC (automatic frequency control) or fly-wheelor Synchrolok circuits, have been devised which greatly reduce thesusceptibility of the horizontal sweep to noise by making use of thefact that the individual horizontal pulses are disturbed at random and,therefore, a running average of the disturbances over severalsynchronizing pulses is disturbed much les than the probable disturbanceof any one pulse.

In these fly-wheel or Synchrolok circuits, 2. local oscillator is usedto develop the synchronizing pulses actually used by the receiverhorizontal sweep circuit. The phase of the oscillation produced by thislocal oscillator is compared with the received synchronizing pulses. Anydiscrepancy or departure from the desired phase relationship produces apulse out of the phase detector for each synchronizing pulse. This trainof pulses has one sign if the phase is leading and the. opposite sign ifthe phase lags. Noise in the synchronizing pulses causes random errorsin the height of these pulses, so that no one pulse is a true measure ofthe phase error of the local oscillator. However, the average value ofthe train of pulses, taken over a large number of these pulses, is avery good measure of the average error for that period. This averagevalue is; obtained by a low-pass filter and the output of this filter isused to control the frequency of the local oscillator by means of areactance tube. If the oscillator is too far advanced in phase, theoutput of the phase detector acting through the 2? low-pass filtercauses the, voltage applied to the reactance. tube to change, and lowersthe oscillator frequency until the phase is correct.

In furtherance of the above objects and in accordance with theinvention, a television receiver synchronizing circuit is provided inwhich the AFC (Synchrolok) circuit controlling the horizontal sweepcircuit is also used to control the vertical sweep circuit. Pulses attwice line scanning frequency are. derived from the local stabilizedoscillator in the AFC circuit. and one of these pulses is selected bythe vertical synchronizing pulse and used to trip the vertical sweepcircuit.

The invention will be, more readily understood referring to the,following description taken in connection with the accompanyingdrawings. iorming a party thereof, in which:

Fig. 1 is a block diagram of a television receiver system employingsynchronizing circuits in accordance with the invention;

Fig. 2 is a circuit diagram or" synchronizing circuits in accordancewith the invention; and

Fig. 3 shows the shapes of the voltage waves at various parts of thecircuit of Fig. 2.

Referring more specifically to the drawings, Fig. 1 shows by way ofexample for purposes of illustration and in block diagram form, atypical television receiver IE1 which has been modified to embody asynchronizin circuit in accordance with the invention. The incomingtelevision signal is received by the antenna II and applied to theapparatus [2 where it is amplified and converted to an intermediatefrequency. The radio frequency amplifier, the converter and theoscillator making up the apparatus l2 are of any wellknown form and neednot be described in detail here. The sound and picture modulatedcarriers are then separated and applied to the corresponding sound andpicture intermediate frequency amplifiers i3 and M, respectively, andthere detected and amplified. The sound is reproduced by the loudspeaker!5 while the picture (video) signal is applied to the video amplifier Itand the output thereof is used to modulate the picture tube IT. Thevideo signal occurring in the output of the video amplifier I6 is alsoapplied to the synchronizing pulse stripper 18, of well-known form. Thefunction of the apparatus I8 is to remove all the picture informationfrom the video signal, thereby leaving only the synchronizing signal.These synchronizing signals are separated into two parts to control thetwo-dimensional picture appearing on the screen of the picture tube IT.For vertical synchronization, the signal is normally integrated and thecircuits are adjusted insuch a way that the-ver- 3 tical sweep isstarted when this signal exceeds a specified level. For horizontalsynchronization there are several different arrangements usedcommercially. One of these is the so-called Synchrolok or fly-wheelsynchronizing circuit and this circuit is utilized in the presentinvention. An exceptionally high degree of stability for the verticalsynchronizing pulse with negligible loss in time is produced byutilizing energy from the already stabilized horizontal synchronizingcircuit to control the vertical sweep circuit.

The output of the synchronizing pulse stripper I8 is applied to theapparatus represented in the dash-dot box 25 in Fig. l (which apparatusis that shown in Fig. 2 and will be described more fully below) and fromthi apparatus horizontal and vertical synchronizing pulses are producedand applied to the horizontal and vertical sweep generators andamplifiers 26 and 21, respectively (which are of well-known form). Inthe members 26 and 21, appropriate sweep waves are generated and thesewaves are applied to the horizontal and vertical deflection coils on theyoke 28 around the neck of the picture tube I! to con trol thedeflection of the beam produced therein in well-known manner.

Reference will now be made to Fig. 2 for a detailed description of theapparatus shown in the large dash-dot box in Fig. l, which comprises aphase detector IS, a low-pass filter 20, a local oscillator 2|, areactance tube circuit 22, a modulator, full-wave rectifier and clipper23 and a low-pass filter 24. The local oscillator 2| is used to developthe synchronizing pulses actually used by the horizontal sweep circuitsin the receiver. The phase of the oscillation produced by this localoscillator is compared with the received synchronizing pulses. Anydiscrepancy or departure from the desired phase relationship produces apulse from the phase detector I9 for each synchronizing pulse. The trainof pulses has one polarity if the phase is leading and the oppositepolarity if the phase lags. Noise in the synchronizing pulses causesrandom errors in the height of these pulses so that no one pulse is atrue measure of the phase error of the local oscillator. However, theaverage value of the train of pulses, taken over a large number ofpulses, is a good measure of the average error for that period. Thisaverage value is obtained by the low-pass filter 20 and the output ofthis filter is used to control the frequency of the local oscillator 2|by means of the reactance tube circuit 22. If the oscillation is too faradvanced in phase, the output of the phase detector l9 acting throughthe low-pass filter 20 causes the voltage applied to the reactance tubeto change. This lowers the oscillator frequency until the phase iscorrect. If the oscillation lags, the opposite effect is product. Beforeexplaining the manner in which the vertical synchronizing pulses arederived, a detailed operation of the so-called Synchrolok circuit willfirst be given. The oscillator 2!, which is a Hartley oscillator,comprises a tube 39 having a coil 3! in its cathode-anode circuit and acoil 32 in its grid-cathode circuit. Connected across the coil 32 is thetuning condenser 33 and resistors 34 and 35, the latter of which can bemade adjustable. Also connected across the serially connected coils 32and EE is the circuit of the reactance tube 22, which will be describedlater. Plate voltage for the tube is obtained by means of any suitablesource 34 through anode resistor 35, while screen voltage is obtained bymeans of the source 36 acting through resistor 31. The screen isby-passed to ground through condenser 38. The circuits of the tube 30are tuned to produce pulses of the correct frequency for horizontalsynchronizing and this synchronization is maintained by the circuit nowto be described.

The received synchronizing signal shown in the drawin (Fig. 3a) andrepresented by the character e1 is applied through coupling condenser40, common resistor 4| and equal resistors 42 and 43 to the cathodes ofthe two phase detector tubes 44 and 45, the anodes of which areconnected together through coil 46, which is closely coupled to coil 41and to coils 32 and 3|. A tuning condenser 48 is connected across thecoil 46. The voltage across this coil is represented in the drawing(Fig. 30) by the reference character e3. The sine wave voltages from theoscillator 2|, applied to the plates of the tubes 44 and by means of thecoupling between the coils 32 and 3| and 46, are equal in amplitude andopposite in polarity. Synchronizing pulses e1 derived from the incomingsignal by means of the pulse stripper l8 are applied to the condenser 40and resistance 4| which diiierentiate the signal applied to the centertap 49 of the winding 46. Horizontal synchronizing pulses thus appear(in the same polarity and of equal amplitude) on the anodes of both ofthe diodes 44 and 45. If the time of arrival of each synchronizing pulsecoincides with the passing of the sine wave through its zero axis, bothdiodes conduct equal current through their equal load resistors 42, 43.The potentials appearing across the individual resistor 42, 43 areopposite in polarity and, therefore, their sum is normally zero, but ifthe phase of a synchronizing pulse changes with respect to that of thesine wave from the oscillator 2!, one diode 44 or 45 will conduct morecurrent than the other. The voltage attained cross the two resistors 42and 43 will not then add to zero, being of negative polarity if one-halfconducts more heavily or of positive polarity if the other half i moreconductive. The output voltage of this phase detector circuit can swingfrom negative through zero to positive (and vice versa) depending on thephase relationship of the incoming synchronizing signal (61) and thesine wave output of the local oscillator 2 I. The output voltage of thephase detector is applied to the control grid of the tube in thereactance tube circuit 22 through an RC circuit including resistor 56and condensers 5'! and 59. This RC network makes up the low-pass filter20 and it is designed to attenuate rapid changes in the direct currentpotential from the phase detector circuit (9, such as are produced byvertical synchronizing pulses or bursts of noise.

The reactance tube 55 with its associated circuit is a variablereactance connected across the coils 32 and 3| of the oscillator tube30. A change of grid potential of the tube 55 produces a change of theconductance of this tube which changes the frequency of the oscillator.If the phase of the oscillator output shifts with respect to that of thesynchronizing pulse (er), the corresponding change in direct currentfrom the phase detector [9 acts to bring the oscillator 2| back intophase. Tube 55 has a cathode resistor 58 and this provides some localfeedback. Plate power is provided from the source 66 through a resistor6| while screen voltage is provided from the source 62 through resistor63. A by-pass condenser 64 is connected between the screen grid andground. The output circuit of the tube 55 is connected across the coils32, 3| of the oscillator 2| by means of the coupling condenser 65. Theoutput of the oscillator 2| is applied to the horizontal sweep generatorand amplifier 26 by means of the coupling condenser 61.

The coil 46 is coupled to the coil 61 which is connected between thecathodes of the balanced diodes l0 and H the anodes of which areconnected together and to the vertical sweep generator and amplifier 27.The tubes and H and their associated circuits form the mixer and fullrectifier represented by the box 23 in Fig. 1. In addition to the sinewaves of horizontal or line scanning frequency applied, in push-pullmanner, to the cathodes of the tubes Hi and II by means of the coil 41,another input voltage e2 is applied to the two cathodes in the samephase. This voltage 62, shown in Fig. 3b, is obtained from thesynchronizing signal e1 after it has been passed through the low-passfilter 24 comprising the series coil 80 and the shunt condenser 8|. Thislow-pass filter attenuates all frequencies of line frequency or higher.The output of this filter, voltage wave e2, does not contain thehorizontal synchronizing pulses nor the equalizing pulses except perhapsas a negligible ripple, and consists simply of a smooth long pulse withsloping sides caused by the vertical synchronizing pulse. All noiseabove the line frequency contained in the synchronizing pulses (61) isgreatly attenuated so that very little noise power is present in theoutput (e2). This output voltage is applied to the mid-terminal 16 ofthe coil 41 (which is magnetically coupled, as mentioned above, to theoscillator coils 32 and 3| in the .Synchrolok circuit). Thismid-terminal is also connected through the resistor 12 to ground. Thecathodes of the two tubes are connected together through the condenser13. The two diodes l0 and H and their associated circuit elements act asa full wave rectifier of the line frequency voltage appearing across thecoil 41. For example, the plates of the diodes 10 and H are held, byconduction of the current through one or the other, to the potential ofwhichever of the cathodes is the more negative. Since the plates of thetubes 10 and 'H are connected through a resistor M to the positiveterminal of source the negative terminal of which is connected toground, the current produced by source 15 flows through the resistance14, through one or the other of the diodes Ill and H to thecorresponding half of the coil 41 and through the resistance 12 toground. The output voltage (es) shown in Fig. 3d as applied to thevertical sweep generator 21 is thus the sum of the voltage e2 (thefiltered vertical synchronizing pulse) and a series of negative halfsine waves generated by the diodes 10 and II plus the voltage e:.. Thevoltage es accordingly contains a series of positive peaks or cuspsoccurring at twice the line frequency. By choosing the capacitance 13 tohave the proper value, the voltage (64) can be so phased that one ofthese positive cusps in ca occurs just before the leading edge of thepulse H0 in the voltage (e2) shown in Fig. 312, while the next oneoccurs just after this edge. This latter cusp and a few following arethus more positive than their neighbors, since the pulse H0 in (ca) actsas a pedestal. The voltage threshold used to trip the vertical sweepcircuit is adjusted so that the first cusp on the pedestal trips thiscircuit. The timing of the vertical sweep is thus controlled by a pulsewhich is itself accurately 6 timed to within a small fraction of thetime of one line while the vertical synchronizing pulse simply selectsthe proper one of the sharp pulses. The output pulse wave after clippingin an amplifier which may form part of the vertical sweep generator 21is shown in Fig. 3e.

Refinements and modifications depend on the associated circuits and thequality of operation desired. For example, the wave a; (Fig. 30) can beclipped to a square wave, differentiated and then full-wave rectified toform a series of sharp pulses rather than the half sine waves shown. Anintermediate amplifier-limiter can be added to develop the wave (67)(Fig. 3 from the pulses in (ca) which exceed the cut-off bias of theamplifier. This would be desirable if a longer pulse, such as that shownby (e7), is required to trip the vertical sweep circuit.

Various other changes can be made in the embodiments described abovewithout departing from the spirit of the invention, as will be obviousto all those skilled in the art.

What is claimed is:

1. In a television receiver, means for receiving television signalscontaining horizontal and vertical synchronizing pulses, means forremoving said horizontal synchronizing pulses from said signals toproduce a residual signal, a local oscillator, a vertical sweep circuit,means responsive to the horizontal synchronizing pulses for stabilizingthe frequency of said local oscillator, means responsive to oscillationsfrom said local oscillator for deriving sharp pulses having a frequencyof recurrence which is twice line scanning frequency, and means formixing said residual signal and said sharp pulses for actuating saidvertical sweep circuit.

2. In a television receiver, a vertical sweep circuit, means forreceiving television signals containing vertical and horizontalsynchronizing pulses, a local oscillator, means for stabilizing thefrequency of said local oscillator by means of said horizontalsynchronizing pulses, means for deriving from said local oscillatorpulses at twice line scanning frequency, and means for selecting one ofsaid pulses at twice line scanning frequency by a vertical synchronizingpulse to actuate the vertical sweep circuit.

3. In a television receiver, a vertical sweep circuit, means forreceiving television signals containing vertical and horizontalsynchronizing pulses, a local oscillator, means for stabilizing thefrequency of said local oscillator by means of said horizontalsynchronizing pulses, means for deriving fro-m said local oscillatorpulses at twice line scanning frequency, and means for selecting one ofsaid pulses at twice line scanning frequency by a vertical synchronizingpulse to actuate the vertical sweep circuit, said means for producingpulses at twice line scanning frequency comprising a full-wave rectifierfor rectifying the output waves from said local oscillator.

BERNARD M. OLIVER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,245,409 lVIiller June 10, 19412,332,681 Wendt Oct. 26, 1943 2,358,545 Wendt Sept. 19, 1944 2,459,699Hallmark Jan. 18, 1949

